What Causes Engine Shaking When Starting a Car?

[DelMiguel]

Engine shaking when starting a car is caused by severe carbon buildup in the engine or ignition system failure. Below is a detailed explanation of the causes of engine shaking during startup: 1. Severe carbon buildup in the engine: When there is excessive carbon buildup inside the engine, the gasoline sprayed by the cold start injector is largely absorbed by the carbon deposits, resulting in an overly lean air-fuel mixture during cold starts. This makes the engine difficult to start. In such cases, the engine can only start easily once the carbon deposits have absorbed enough gasoline to reach saturation. After starting, the gasoline adsorbed on the carbon deposits is then drawn into the combustion chamber by the engine's vacuum suction, enriching the air-fuel mixture again. This fluctuation between lean and rich air-fuel mixtures causes the engine to idle roughly after a cold start. 2. Ignition system failure: Issues with the ignition system or poorly functioning spark plugs can also lead to such symptoms.